In wire bonding for manufacturing semi-conductor packages, inner leads of a lead frame should closely contact a heat block so as to firmly maintain the electric connection between inner leads and a semiconductor chip and thus obtain a good yield of semi-conductor packages.
Conventionally, there have been proposed such clamping devices adapted to closely contact inner leads of a lead frame to a heat block. A typical example of conventional clamping devices is illustrated in FIG. 1.
FIG. 1 is a perspective view showing a known wire bonding process wherein a semi-conductor chip 4 which has been fixedly attached to a paddle 3 of a lead frame 2 supported on the upper surface of a heat block 1 are connected with inner leads 5 of the lead frame 2, by means of wires, particularly gold wires 6. In this case, the clamping device for inner leads comprises an inner lead clamp 7 and an auxiliary clamp 11 which cooperate with each other to press totally a plurality of inner leads 5 against a heat block 1.
The inner lead frame 7 is of a plate shape having a certain thickness and has a generally rectangular working opening 8. A pair of support slots 9 are formed at respective middle portions of both short side edges of the working opening 8. On the lower surface of inner lead clamp 7, a pressing protrusion 10 having a rectangular rim with a certain thickness is formed around the peripheral edges of the working opening 8. On the other hand, the auxiliary clamp 11 comprises a pair of spaced horizontal portions 12 and a pair of curved vertical portions 13 extending downward from respective ends of the horizontal portions 12. The vertical portions 13 of auxiliary clamp 11 are fitted in the support slots 9 of inner lead clamp 7, respectively.
In wire bonding, the inner lead clamp 7 is positioned on the inner leads 5 of lead frame 2 which has been previously heated. Under the condition, the vertical portions 13 of auxiliary clamp 11 are fitted in both support slots 9 of inner lead clamp 7, respectively. Thereafter, the auxiliary clamp 11 is properly depressed by the worker's force so that inner leads 5 of the lead frame 2 are closely in contact with the heat block 1. Under the condition, respective inner leads 4 are connected with the semi-conductor chip 4, by means of wires 6.
However, this conventional inner lead clamp device has a disadvantage that inner leads 5 can not be firmly clamped, because of using the inner lead clamp 7 shaped into an integrated type having a rectangular rim shaped pressing protrusion 10 adapted to press several inner leads 5 totally and the auxiliary clamp 11 adapted to press the inner lead clamp 7 so as to closely contact the inner leads 5 to the heat block 1.
That is, although the pressing protrusion 10 of the inner lead clamp 7 presses several inner leads 5 simultaneously, a part of inner leads 5 may not be closely in contact with the heat block 1, because a certain gap may be formed between inner leads 5 and the heat block 1, due to a possible deformation of inner leads 5 occurring in its machining and its transportation, an imprecise machining of and a poor maintenance of the heat block 1, a possible deformation of the inner lead clamp 7 occurring in its machining and assembling and a possible uneven pressing force applied to the inner lead clamp 7. As a result, the connection of wires 6 is unstable, thereby causing the reliability of semi-conductors to be decreased.